Adsorption techniques have been used in the separation and in the production of a variety of gases including hydrogen, helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen, and nitrogen. Feed gases useful in these adsorption separations include air; refinery off-gases; and land-fill, flue, and natural gases.
Pressure swing adsorption (PSA) processes typically involve the cyclical pressurization, production, and regeneration of adsorbent beds or treatment zones to achieve a product of a specified purity. The pressurization step is usually carried out at a constant rate. This type of system is graphically illustrated in FIGS. 1-3. FIG. 1 represents a constant pressure pressurization step. FIG. 2 corresponds to the feed flow rate corresponding to the pressurization history in FIG. 1. FIG. 3 represents the hypothetical concentration profile of the impurities in the interstitial gas at the end of the pressurization step corresponding to FIG. 1, as a function of bed length. The feed rate for the constant pressure history is flat, and therefore, the final concentration profile should be higher in purity at the production end and lower at the injection end.
However, such processes for the production of inert gases having an oxygen content below 100 vpm (0.01%) from air are inefficient. PSA designs having multiple treatment zones are more effective for obtaining highly pure inert gases, but the cost for such systems are much higher than traditional two-zone PSA plants. The purity of a gas product of a PSA process is determined by the differences in the uptake rates of the feed gas components which in turn is dependent upon the length of time the feed gas is in contact with the adsorbent in the treatment zone. The initial product coming out of the treatment zone has the shortest residence time and is therefore of the lowest purity. The purity expectedly improves with time and then decreases as the adsorption wave approaches the bed exit.
A catalytic combustion unit is occasionally combined with a PSA plant when very low oxygen concentration is required. The PSA plant is operated to produce inert gas with oxygen concentrations between 10,000 and 1,000 vpm (1.0 and 0.1 percent, respectively) which is then combusted to produce a final inert gas containing 100 vpm or less of oxygen. This operation is costly, however, as the catalytic units and the fuel used in the combustion are expensive. Furthermore, it is necessary to remove the combustion products from the effluent of the catalytic unit.
Armond et al., U.S. Pat. No. 4,144,037, disclose a process for gas separation wherein the gaseous mixture is drawn through an absorbent bed in a substantially unpressurized condition by applying reduced pressure to the outlet of the bed.
Leitgeb, U.S. Pat. No. 4,256,465, offsets the initial low purity surge of product gas of a typical constant pressurization PSA system by slowing the PSA process concentration wave as it moves through the treatment zone. This was done by multi-stage pressurization where the final stage was conducted substantially more slowly than the initial stage. The Leitgeb process actually makes use of a lengthy adsorption time before product is taken though, and does not depend on the pressurization rates.
This type of a system is graphically illustrated in FIGS. 4-6. FIG. 4 represents a rapid initial pressurization followed by a slow pressurization at the end of the pressurization step. FIG. 5 corresponds to the feed flow rate corresponding to the pressurization history in FIG. 4. FIG. 6 represents the hypothetical concentration profile of the impurities in the interstitial gas at the end of the pressurization step corresponding to FIG. 4, as a function of bed length. The feed rate for this "early" pressure history is initially so fast that the bed comes up to pressure before a lot of the feed gas is adsorbed. Consequently, the resultant final concentration history is flat, and the purity of the initial product is low. Therefore, a PSA plant operating with an early feed history on a given cycle and at specified feed and product rates would produce the lowest purity.
Ward, U.S. Pat. No. 4,572,723, discloses a process for the production of low oxygen content nitrogen product gas wherein the cycle time is preferably 500 seconds and the pressure is allowed to rise at a constant slow rate. U.K. Patent Application No. 2 195 097 describes a PSA gas separation wherein the product gas returns whenever the pressure in a product gas reservoir exceeds that of the treatment zone.
Japanese Patent Publication No. 63-79714 discloses a three treatment zone merry-go-round PSA system wherein at any given time, two serially connected treatment zones are used for adsorption while a third treatment zone is regenerated.
The PSA processes of the present invention alleviate the problems of the prior art by the cost-effective production of substantially pure product gas by a two step slow/fast pressurization with feed gas or by product gas pressurization in combination with one step or two step slow/fast feed gas pressurization.